$-\dfrac{3}{8} + \dfrac{9}{5} = {?}$
Answer: $ = - {\dfrac{3 \times 5}{8 \times 5}} + {\dfrac{9 \times 8}{5 \times 8}} $ $ = - {\dfrac{15}{40}} + {\dfrac{72}{40}} $ $ = - \dfrac{{15} + {72}}{40} $ $ = \dfrac{57}{40}$